The present invention relates generally to solenoid valves and more particularly, to solenoid valves of the type arranged between an actuator housing and a cover member in a antilock brake system (hereafter refer to as ABS) for a motor vehicle.
Conventionally, in view of the control relative to pressure increase and decrease and requirements of holding pressure at a constant value, some ABS include a normally open solenoid valve arranged in a brake fluid passage for conducting hydraulic pressure produced in a master cylinder (hereafter refer to as M/C) to a wheel cylinder (hereafter refer to as W/C), and a normally closed solenoid valve arranged in a return passage from W/C. Such ABS may be constructed to enable the individual control of three systems such as a right front wheel, a left front wheel, and right and left rear wheels, including an integrated actuator unit of a housing having six solenoid valves and fluid passages, various members, etc. thereof built therein, and a housing cover. See, for example, JP-A 3-167061.
With such known actuator unit, however, since in a state that a valve function part of each solenoid valve is hermetically fitted in a bore of the housing, the housing cover is disposed on a top of a solenoid part of the solenoid valve, and fixed to the housing by a plurality of bolts, tightening force of the bolts directly operates on each solenoid valve.
As a result, during soft pressure increase and soft pressure decrease of the ABS control, surging occurs due to overshoot by sudden variations of hydraulic pressure. In case of soft pressure increase, for example, surging as shown in FIG. 3 occurs. Referring to FIG. 3, surging generating phenomena will be described. Suppose that the solenoid valve is now in the ON state to hold hydraulic pressure supplied to W/C at a constant value. In this state, if a pulse signal is generated to control the solenoid valve to be OFF. instantaneous pressure increase is produced corresponding to the width of the pulse signal. Since an action of the solenoid valve has time lags t.sub.1, t.sub.2 upon rise and fall of the pulse signal, surging with the maximum damping width as indicated by .DELTA.P2 occurs with respect to an expected soft pressure increase amount .DELTA.P1. Likewise, surging occurs upon soft pressure decrease. Such surging is transmitted to a vehicular body, causing uncomfortable vibration noise.
It is, therefore, an object of the present invention to provide solenoid valves which contribute to a restraint of surging upon soft pressure increase and soft pressure decrease.